1. Field of the Invention
The present invention relates to a magnetoresistive element and a method of manufacturing the same, and to a thin-film magnetic head, a head assembly and a magnetic disk drive each including the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write head having an induction-type electromagnetic transducer for writing and a read head having a magnetoresistive element (that may be hereinafter referred to as MR element) for reading are stacked on a substrate.
MR elements include GMR (giant magnetoresistive) elements utilizing a giant magnetoresistive effect, and TMR (tunneling magnetoresistive) elements utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, those incorporating spin-valve GMR elements or TMR elements have been mass-produced.
Spin-valve GMR elements and TMR elements each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the spacer layer. The free layer is a ferromagnetic layer having a magnetization that changes its direction in response to a signal magnetic field. The pinned layer is a ferromagnetic layer having a magnetization in a fixed direction. The antiferromagnetic layer is a layer that fixes the direction of the magnetization of the pinned layer by means of exchange coupling with the pinned layer. The spacer layer is a nonmagnetic conductive layer in spin-valve GMR elements, or is a tunnel barrier layer in TMR elements.
Read heads incorporating GMR elements include those having a CIP (current-in-plane) structure in which a current used for detecting a signal magnetic field (hereinafter referred to as a sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element, and those having a CPP (current-perpendicular-to-plane) structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element.
Read heads each have a pair of shields sandwiching the MR element. The distance between the two shields is called a read gap length. Recently, with an increase in recording density, there have been increasing demands for a reduction in track width and a reduction in read gap length in read heads.
As an MR element capable of reducing the read gap length, there has been proposed an MR element including two ferromagnetic layers each functioning as a free layer, and a spacer layer disposed between the two ferromagnetic layers (such an MR element is hereinafter referred to as an MR element of three-layer structure), as disclosed in U.S. Pat. No. 6,914,759 B2 and U.S. Pat. No. 7,035,062 B1, for example. In the MR element of three-layer structure, the two ferromagnetic layers have magnetizations that are in opposite directions when no external magnetic field is applied to the layers and that change directions in response to an external magnetic field.
A typical configuration and function of a read head incorporating an MR element of three-layer structure will now be described. The MR element of three-layer structure has a front end face located in a medium facing surface, and a rear end face opposite to the front end face. The read head incorporating the MR element of three-layer structure includes a bias magnetic field applying layer that is adjacent to the rear end face of the MR element with an insulating layer located therebetween, and that applies a bias magnetic field to the two ferromagnetic layers. The bias magnetic field applying layer is composed of a permanent magnet made of a hard magnetic material, for example. The bias magnetic field changes the directions of the magnetizations of the two ferromagnetic layers so that the directions of their magnetizations each form an angle of approximately 45 degrees with respect to the direction of track width. As a result, a relative angle of approximately 90 degrees is formed between the directions of the magnetizations of the two ferromagnetic layers. When a signal magnetic field from the recording medium is applied to the read head, the relative angle between the directions of the magnetizations of the two ferromagnetic layers changes, and as a result, the resistance of the MR element changes. With this read head, it is possible to detect the signal magnetic field by detecting the resistance of the MR element.
Read heads incorporating MR elements of three-layer structure allow a greater reduction in read gap length, compared with read heads incorporating conventional GMR elements. For example, in a CPP-structure read head incorporating a conventional GMR element, the read gap length can be reduced to approximately 30 nm at the smallest. In contrast, in a CPP-structure read head incorporating an MR element of three-layer structure, the read gap length can be reduced to approximately 20 nm or smaller.
Meanwhile, read heads incorporating conventional MR elements of three-layer structure have a disadvantage that, when a large number of read heads are manufactured to the same specifications, asymmetry of output of the read heads greatly varies and consequently the yield of the read heads is low. Asymmetry of output of a read head refers to asymmetry between the output of the read head when a magnetic field of +H is applied to the read head in a direction perpendicular to the medium facing surface and the output of the read head when a magnetic field of −H is applied to the read head in the direction perpendicular to the medium facing surface.
Presumably, the reason why the asymmetry of output greatly varies in read heads incorporating conventional MR elements of three-layer structure is that a sufficient bias magnetic field cannot be stably applied to the two ferromagnetic layers of the MR element from the bias magnetic field applying layer that is adjacent to the rear end face of the MR element with an insulating layer located therebetween. The following two factors may be responsible for this. A first factor is that a sufficient bias magnetic field is not obtainable because of a low saturation magnetization of the bias magnetic field applying layer. A second factor is that a reduction in read gap length causes a reduction in distance between the bias magnetic field applying layer and each shield, and consequently the occurrence of leakage of a magnetic field from the bias magnetic field applying layer to the shields becomes noticeable.
JP 10-312512A discloses a technique of providing a bias magnetic field applying film composed of a magnetic underlayer made of a magnetic material having a high saturation magnetization and a hard magnetic layer formed on the magnetic underlayer in a read head incorporating a typical spin-valve GMR element having a free layer, a spacer layer and an antiferromagnetic layer. According to this technique, it is possible to increase the saturation magnetization of the bias magnetic field applying film as a whole.
The technique disclosed in JP 10-312512A, when applied to a read head incorporating an MR element of three-layer structure, cannot sufficiently reduce variations in asymmetry of output of the read head, however. This is presumably because, according to the technique, the occurrence of leakage of a bias magnetic field from the hard magnetic layer to the lower shield is noticeable since the magnetic underlayer made of a magnetic material having a high saturation magnetization is disposed between the hard magnetic layer and the lower shield. It is therefore difficult with the above technique to eliminate the second factor described above.
Generally, the hard magnetic layer constituting the bias magnetic field applying layer has a hexagonal close-packed structure in which the C-axis is the easy axis of magnetization, and it is therefore desirable to align the C-axis in the in-plane direction. To achieve this, it is necessary that the hard magnetic layer be formed on a buffer layer having a body-centered cubic structure. In addition, it is preferable to eliminate crystal lattice mismatch between the buffer layer and the hard magnetic layer. If the technique disclosed in JP 10-312512A is applied here so that the magnetic underlayer and the hard magnetic layer are formed in this order on the buffer layer, the crystallinity and orientability of the hard magnetic layer are degraded compared with the case in which the hard magnetic layer is formed directly on the buffer layer, and as a result, the coercivity of the hard magnetic layer is reduced. If the coercivity of the hard magnetic layer is reduced, variations in output of the read head caused by a disturbance magnetic field increase, or demagnetization of the hard magnetic layer occurs easily upon collision between the magnetic head and the recording medium, for example. As a result, the reliability of a magnetic disk drive including the magnetic head decreases.